A typical fluid treatment filter element, for many uses, comprises a hollow, generally cylindrical, permeable filter element with appropriate end caps to direct a fluid, e.g., a gas or liquid, through the filter medium of the filter element.
The fluid to be treated is generally forced to flow inwardly from the exterior to the interior of such a fluid treatment element, although there is no requirement that such be the case. It should be noted that irrespective of the normal flow of fluid through the fluid treatment element, it is not unusual to have the fluid flow reversed, either accidentally (e.g., due to a surge in fluid pressure downstream from the filter element) or intentionally (e.g., to flush an accumulated cake of particulate matter from the surface of the filter element).
Certain filters are specifically designed to withstand such a reversal of fluid flow. Such filters are commonly referred to as backwashable, or septa, filters. Regardless of the particular application of the filter, e.g., petrochemicals, pharmaceuticals, electronics, or food and beverage preparation, the common feature of such filters is that they must be able to withstand a strong flow in the reverse direction (backwashing) without experiencing a loss in integrity. In addition to maintaining their integrity, such filters should also be resistant to chemical attack in order to be useful over a wide range of applications.
In an effort to address the aforesaid concerns, those in the art turned to filters prepared from stainless steel. Such porous stainless steel filters, however, have several significant disadvantages: they are very expensive, they clog quickly upon use and therefore provide short cycle times, and are difficult to dispose of after use.
In view of the disadvantages associated with stainless steel filters, the use and development of non-metallic, backwashable filters, particularly nonwoven fibrous filtration media, has been on the rise. One such filter is prepared using polypropylene fibers which are wound onto a porous core. Alternatively, such filters are prepared using a fiber impingement process which allows greater control of the fiber diameter and structure of the media prepared from such polypropylene fibers.
Polypropylene backwashable filters, however, possess several disadvantages. Those disadvantages include: the inability to be used in applications where the filter contacts oils or heated liquids such as boiling water, the inability to be used in high temperature applications due to its relatively low melting point (175.degree. F.), and the need for a suitable porous support layer to be wrapped about the polypropylene fibers so that the polypropylene can withstand backwashing without damage. Such an exterior protective material or outerwrap is typically comprised of a metal mesh, such as of aluminum or stainless steel, although a plastic mesh or nonwoven material has also been used. However, the outerwrap presents yet another component of the filter which, depending upon its composition, can degrade or be otherwise damaged upon exposure to the fluid to be filtered.
In view of the foregoing, there remains a need for a filtration medium which is not readily damaged during backwashing, even in the absence of a porous support layer about the outer surface of the medium, can withstand exposure to high pressure drops of high-temperature liquids (in excess of 100.degree. C.), possesses superior resistance to solvents, including oils, and which is nonetheless economical to manufacture and utilize.
It is an object of the present invention to provide such a filtration medium, as well as a filter element and cartridge which comprises such a filtration medium. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.